Influence of potassium on benzene and soot formation in fuel-rich oxidation of methane in a laminar flow reactor

The influence of potassium chloride (KCl) and potassium hydroxide (KOH) on the formation of benzene and soot in fuel-rich oxidation of methane was investigated experimentally using a flow reactor at atmospheric pressure and temperatures ranging from 1273 to 1673 K. The methane inlet concentration was kept constant at 2 vol.% (nitrogen balance), while the C/O ratio was 2 or 5. The potassium salt was fed into the main gas flow at a comparatively high concentration of around 1000 ppm. The onset of formation of acetylene and benzene was around 1373 K. The addition of both potassium salts was found to promote formation of acetylene and benzene at temperatures below 1523 K. This effect was most pronounced at a high C/O ratio of 5 and in the presence of KOH, but diminished at temperatures above 1523 K. A detailed chemical kinetic model was used to interpret the experimental results and was found to be able to partially reproduce the promoting effect of KOH. According to the model, the addition of KOH facilitates the buildup of OH radicals through the rate controlling reaction KOH + M ⇌ K + OH + M. The promotion of the radical pool favors the formation of active intermediates including methyl and vinyl radicals, both of which play an important role in the major benzene formation pathways under the conditions investigated. Particle analysis using SMPS showed a net increase in the volume concentration of soot and a shift of the particle size distribution towards larger diameters in the presence of KOH, but no quantitative difference was observed for the mass of carbonaceous materials collected on filters.